Two-stage rotodynamic blood pump

ABSTRACT

A pump ( 10 ) includes a housing, a stator ( 20 ) supported in the housing, and a rotor assembly ( 30 ). The rotor assembly ( 30 ) includes a rotor ( 32 ) supported in the housing for rotation relative to the stator ( 20 ) about an axis ( 12 ). The rotor assembly ( 30 ) also includes a first impeller ( 34 ) operatively coupled to a first axial end of the rotor ( 32 ) for rotation with the rotor about the axis ( 12 ). The rotor assembly further includes a second impeller ( 36 ) operatively coupled to a second axial end of the rotor ( 32 ), opposite the first axial end, for rotation with the rotor about the axis ( 12 ). The rotor assembly ( 30 ) is movable along the axis ( 12 ) relative to the housing to adjust hydraulic performance characteristics of the pump ( 10 ).

RELATED APPLICATION

This application claims priority from U.S. provisional patentapplication Ser. No. 60/795,096, filed on Apr. 26, 2006, the subjectmatter of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a pump that may be used in fluidhandling applications where two fluid streams are to be balanced. Moreparticularly, the present invention relates to a two-stage rotodynamicpump configuration for providing pulsatile, continuous flow, bloodpumping performance.

BACKGROUND OF THE INVENTION

Congestive heart failure (CHF) is an increasingly common cause ofcardiovascular disability and premature death. Despite advances inmedical therapy, heart transplant is the primary course of action fortreating patients with end-stage congestive heart failure. Because theavailability of donor organs is limited, however, CHF patients may beforced to wait until a suitable donor organ is located. Blood pumpingdevices, referred to as ventricular assist devices (VADs) and totalartificial hearts (TAH), can be used as a bridge-to-transplant option inorder to save patients with CHF and other cardiac conditions whootherwise would not survive until a suitable donor organ is located.Ultimately, such blood pumping devices will become viable as permanentor long-term alternatives to transplant.

SUMMARY OF THE INVENTION

The present invention relates to a valveless, sensorless, pulsatile,continuous flow total artificial heart that can self balance left andright circulation, without electronic intervention, by acting as aninlet pressure balancing regulator as it pumps. Left and rightcirculations are impelled via a single moving part, which revolveswithin a brushless, sensorless DC motor winding. This rotating assemblyis free to move axially in response to the hydraulic environment,thereby changing clearances in the two opposed rotodynamic pumpingstages, affecting relative performance to balance the inlet pressures.In an alternate embodiment, external electronic control is employed tocontrol the position of the rotating assembly via an electromotiveforce, such as a solenoid-type element. The pump configurations of thepresent invention may also be applied to other fluid handlingapplications where inlet pressure balancing is desired.

The present invention relates to a blood pump that includes a housing, astator supported in the housing, and a rotor assembly. The rotorassembly includes a rotor supported in the housing for rotation relativeto the stator about an axis. The rotor assembly also includes a firstimpeller operatively coupled to a first axial end of the rotor forrotation with the rotor about the axis. The rotor assembly furtherincludes a second impeller operatively coupled to a second axial end ofthe rotor, opposite the first axial end, for rotation with the rotorabout the axis. The rotor assembly is movable along the axis relative tothe housing to adjust hydraulic performance characteristics of the pump.

The present invention also relates to a blood pump that includes a motorthat includes a stator and a rotor rotatable about an axis relative tothe stator. A first pump stage includes a first pump housing and a firstimpeller rotatable with the rotor about the axis in the first pumphousing. A second pump stage includes a second pump housing and a secondimpeller rotatable with the rotor about the axis in the second pumphousing. The blood pump is adapted to adjust the axial position of thefirst impeller in the first housing and the axial position of the secondimpeller in the second housing to adjust hydraulic performancecharacteristics of the first and second pump stages. Axial movement ofthe first and second stages is equal and opposite.

The present invention also relates to a blood pump that includes a motorcomprising a stator and a rotor rotatable about an axis relative to thestator. The blood pump also includes a first pump stage comprising afirst pump housing and a first impeller rotatable with the rotor aboutthe axis in the first pump housing. The blood pump further includes asecond pump stage comprising a second pump housing and a second impellerrotatable with the rotor about the axis in the second pump housing. Thefirst pump stage is configured to have a pressure rise that decreasessharply with increasing flow; the first pump stage flow thus beingprimarily a function of pump speed and impeller position. The secondpump stage is configured to have a pressure rise that is primarily afunction of pump speed and impeller position and substantiallyindependent of flow.

The present invention also relates to a pump including a housing thatdefines first and second pump housings. A rotor is supported in thehousing and rotatable about an axis. The rotor includes a first impellerdisposed in the first pump housing and a second impeller disposed in thesecond pump housing. The pump is configured such that inlet pressuresacting on the first impeller move the rotor relative to the housing in afirst direction along the axis and inlet pressures acting on the secondimpeller move the rotor relative to the housing in a second directionalong the axis opposite the first direction.

The present invention further relates to a pump including a housingincluding a pumping chamber and a rotor supported in the housing androtatable about an axis. The rotor includes an impeller at leastpartially disposed in the pumping chamber. The rotor is movable relativeto the housing in an axial direction parallel to the axis. The pump isconfigured such that axial movement of the rotor causes the impeller tomove axially between the pumping chamber and an adjacent chamber toalter the hydraulic performance of the pump.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 is a perspective view of a blood pump according to a firstembodiment of the present invention;

FIG. 2 is a sectional view of the blood pump taken generally along line2-2 in FIG. 1;

FIG. 3 is an exploded view of the blood pump;

FIGS. 4 and 5 are plan views of portions of the blood pump;

FIG. 6 is a sectional view illustrating a blood pump according to asecond embodiment of the present invention;

FIG. 7 is a sectional view illustrating a blood pump according to athird embodiment of the present invention; and

FIG. 8 is a graph illustrating performance characteristics of the bloodpump of FIG. 7;

FIG. 9 is a perspective view of a blood pump according to a fourthembodiment of the present invention;

FIG. 10 is a front view of the blood pump of FIG. 9;

FIG. 11 is a perspective view of a portion of the blood pump of FIG. 9;

FIG. 12 is a sectional view of the blood pump taken generally along line12-12 in FIG. 9; and

FIG. 13 is a graph illustrating performance characteristics of the bloodpump of FIG. 9.

DESCRIPTION OF EMBODIMENTS

The present invention relates to a blood pump. FIG. 1 illustrates ablood pump 10 according to a first embodiment of the present invention.According to the present invention, the blood pump 10 is a totalartificial heart (TAH) device capable of replacing a failing or damagedhuman heart. Those skilled in the art, however, will appreciate that theblood pump 10 could be suitable for non-TAH implementations, such asbiventricular support. Those skilled in the art will also appreciatethat the pump may be suited for purposes other than pumping blood, suchas any implementation in which a dual or two stage fluid handling pumpwith pressure balancing features is desired. In the illustratedembodiments, the blood pump 10 is a two-stage centrifugal pump, which isdescribed below in further detail. The blood pump 10 could, however, bea rotodynamic pump of any desired configuration.

Referring to FIGS. 1-3, the blood pump 10 includes a stator assembly 20,a rotor assembly 30, a left pump housing 40, and a right pump housing50. In an assembled condition of the blood pump 10 (FIGS. 1 and 3), therotor assembly 30 is supported by the stator assembly 20 for rotationabout an axis 12. The pump housings 40 and 50 are fixed to the statorassembly 20 to enclose the rotor assembly 30. The rotor assembly 30includes a motor rotor 32, a first or left impeller 34, and a second orright impeller 36.

The motor rotor 32 includes a core 60 (FIG. 2) upon which a ring-shapedpermanent magnet 62 is mounted. A low density magnetically permeablefill material 64 may be used to support the magnet 62 on the motor rotor32, thereby allowing a neutral buoyancy rotating assembly, andinsensitivity to pump assembly attitude. The left and right impellers 34and 36 are secured to the core 60 by known means, such as adhesives ormechanical fasteners. Alternatively, the impellers 34 and 36 could beformed (e.g., molded) as a single piece of material with the core 60.

The stator assembly 20 includes a stator housing 22 that supports amotor stator 24. The motor stator 24 includes a stator core and motorwindings, illustrated schematically at 26 and 28, respectively in FIG.2. The motor windings 28 are electrically connected to three controlwires 70 of a control cable 72 that enters the stator housing 22 througha conduit 74 and is sealed by a potting material 76.

The blood pump 10, when assembled, includes a centrifugal first or leftpumping stage or pump 42. The left pump 42 includes the left impeller 34and a left pump chamber 44 in which the left impeller is disposed. Theleft pump chamber 44 is defined, at least partially, by the left pumphousing 40 and the stator assembly 20. The left pump 42 also includes aleft pump inlet 46 and a left pump outlet 48 that, in the illustratedembodiment, are formed as integral portions of the left pump housing 40.The left pump housing 40 includes an inlet surface 90 that helps definean inlet portion 92 of the left pump chamber 44 in fluid communicationwith the inlet 46. The left pump housing 40 also includes a volutesurface 94 that helps define a volute portion 96 of the left pumpchamber 44 in fluid communication with the outlet 48.

The blood pump 10, when assembled, also includes a centrifugal second orright pumping stage or pump 52. The right pump 52 includes the rightimpeller 36 and a right pump chamber 54 in which the right impeller isdisposed. The right pump chamber 54 is defined, at least partially, bythe right pump housing 50 and the stator assembly 20. The right pump 52also includes a right pump inlet 56 and a right pump outlet 58 that, inthe illustrated embodiment, are formed as integral portions of the rightpump housing 50. The right pump housing 50 includes an inlet surface 100that helps define an inlet portion 102 of the right pump chamber 54 influid communication with the inlet 56. The right pump housing 50 alsoincludes a volute surface 104 that helps define a volute portion 106 ofthe right pump chamber 54 in fluid communication with the outlet 58.

The motor rotor 32 and motor stator 24 help define a motor 80 of theblood pump 10 that drives the left and right pumps 42 and 52. The motor80 may be any type of electric motor suited to drive the pumps 42 and 52and deliver the desired performance characteristics. For example, in theillustrated embodiment, the motor 80 may have a single phase ormulti-phase brushless, sensorless DC motor configuration. A motorcontroller 82 is operative to excite the phase windings 28 of the motor80 via the cable 72 to achieve desired performance of the motor portion,such as motor speed or current. For example, the motor controller 82apply pulse width modulated voltage to the motor phases in order toachieve the desired motor/pump performance.

During operation of the blood pump 10, the rotor assembly 30 rotatesabout the axis 12 relative to the stator assembly 20. The rotor assembly30 is supported or rides on a hydrodynamic or fluid film bearing formedby the pumped fluid, i.e., blood. Alternatively, the blood pump 10 couldinclude other types of bearing features, such as mechanical bearings orbearing surfaces formed from or coated with low friction materials, forfacilitating rotation of the rotor assembly 30. As a furtheralternative, the rotor assembly 30 could be magnetically suspended.

The materials used to construct the blood pump 10 may be formed frommaterials conducive to blood pumping implementations. For example,portions of the blood pump 10 that are exposed to blood flow during use,such as the impellers 34 and 36 and pump housings 40 and 50, may beformed from, coated, or encased in a biocompatible material, such asstainless steel, titanium, ceramics, polymeric materials, compositematerials, or a combination of these materials. Surfaces or portions ofthe blood pump 10 that may contact each other during use, such as theleft impeller 34 and pump housing 40 or the right impeller 36 and pumphousing 50, may also be formed or coated with low friction materials,such as a fluorocarbon polymer coatings, diamond-like carbon coatings,ceramics, titanium, and diamond coated titanium.

Referring to FIG. 1, arrows are used to illustrate the blood pump 10 ina total artificial heart (TAH) implementation in which the pump takesover the function of a patient's heart (not shown). In thisconfiguration, the left pump inlet 46 is connected with the left atrium,the left pump outlet 48 is connected to the aorta, the right pump inlet56 is connected to the right atrium, and the right pump outlet 58 isconnected to the pulmonary artery. In operation, the left pump 42delivers oxygenated blood to the aorta from the left atrium and theright pump 52 delivers deoxygenated blood to the pulmonary artery fromthe right atrium.

Those skilled in the art will appreciate that, in a TAH scenario, it isimportant to balance pulmonary and systemic arterial blood flows andatrial pressures. For example, if the right pump 52 delivers blood at ahigher flow rate than the left pump 42, blood may accumulate in thelungs and can lead to congestive heart failure. For example, if the leftpump 42 delivers blood at a higher flow rate than the right pump 52,blood may accumulate in the liver and can lead to liver failure. Thegoal for the blood pump 10 is thus to balance pulmonary and systemicarterial blood flows and atrial pressures. According to the presentinvention, the blood pump 10 balances systemic and pulmonary atrialpressures and arterial flow rates by adjusting the geometry orconfiguration of the left (systemic) pump 42 and right (pulmonary) pump52.

According to the present invention, the blood pump 10 is configured witha clearance that permits axial movement of the rotor assembly 30relative to the stator assembly 20. Referring to FIG. 2, the rotorassembly 30 is positioned about midpoint in this axial clearance,leaving an axial back clearance between the left impeller 34 and thestator housing 22, identified generally at “A1,” and an axial backclearance between the right impeller 36 and the stator housing 22,identified generally at “A2.” With the configuration shown in FIG. 2, ithas been found that maximum left pump 42 performance occurs when A1 isminimum, and maximum right pump 52 performance occurs when A2 isminimum. During operation of the blood pump 10, the rotor assembly 30can move or shuttle axially relative to the stator assembly 20 due tohydrodynamic pumping forces created by the left and right pumps 42 and52. The rotor assembly 30 can move axially between a left position, inwhich the left impeller 34 is positioned with A1 being maximum, and aright position, in which the right impeller 36 is positioned with A2being maximum.

When the rotor assembly 30 moves axially between the left and rightpositions, the configurations or geometries of the left and right pumps42 and 52 are altered. As the axial position of the left impeller 34changes, the clearance A1 between the left impeller and the statorassembly 22 changes, which alters the configuration and geometry of theleft pump 42 and left pump chamber 44. Similarly, as the axial positionof the right impeller 36 changes, the clearance A2 between the rightimpeller and the stator assembly 22 changes, which alters theconfiguration and geometry of the right pump chamber 54 and theconfiguration or geometry of the right pump 52.

As the clearances A1 and A2 increase, the first and second pumps 42 and52 decrease in hydraulic output. Thus, for a given pump speed, as theimpellers 34 and 36 move toward the stator assembly 22 (i.e., reducingtheir respective clearances A1 and A2), the pumps 42 and 52 increase inpressure and flow accordingly. Conversely, as the impellers 34 and 36move away from the stator assembly 22 (i.e., increasing their respectiveclearances A1 and A2), the pumps 42 and 52 decrease in pressure and flowaccordingly.

It will thus be appreciated that, for the single motor, two-stageconfiguration of the blood pump 10 of the present invention, axialmovement of the rotor assembly 30 that produces increased pressure andflow at the left pump stage 42 will also produce a decrease in pressureand flow at the right pump stage 52. Similarly, axial movement of therotor assembly 30 that produces increased pressure and flow at the rightpump stage 52 will also produce a decrease in pressure and flow at theleft pump stage 42. From this, it follows that, for any given speed ofthe blood pump 10, the pressures and flows of the left and right pumpstages 42 and 52 can be balanced if the axial position of the rotorassembly 30 relative to the stator assembly 20 is adjusted to the properposition.

Based on this principle, using the blood pump 10, systemic and pulmonarypressure and flow characteristics can be controlled through adjustingthe axial position of the rotor assembly 30. According to the presentinvention, the axial position of the of the rotor assembly 30 can becontrolled passively or actively. The embodiment of FIGS. 1-5illustrates a configuration of the blood pump 10 in which passivecontrol is used to adjust the axial position of the rotor assembly 30and, thus, the geometry or configuration of the left and right pumps 42and 52.

In the passive control configuration of the blood pump 10, the axialposition of the rotor assembly 30 is controlled passively or inherentlythrough hydraulic forces created by the left and right pumps 42 and 52during operation. According to the present invention, the configurationsof the left and right impellers 34 and 36 are chosen to help producethis operation. Referring to FIG. 4, the first impeller 34 includes aback plate 110 and a plurality of vanes 112 that extend radially fromthe back plate. In the embodiment of FIG. 4, the vanes 112 include firstor primary vanes 114 and second or splitter vanes 116, the splittervanes being shorter than the primary vanes. In the embodimentillustrated in FIG. 5, the vanes 112 are configured with a low incidenceinlet and a radial discharge.

Referring to FIG. 5, the second impeller 36 includes a back plate 120and a plurality of vanes 122 that extend radially from the back plate.In the embodiment of FIG. 5, the vanes 122 include first or primaryvanes 124 and second or splitter vanes 126, the second vanes beingshorter than the first vanes. In the embodiment illustrated in FIG. 5,the vanes 122 are configured with a low incidence inlet and a radialdischarge.

The back plates 110 and 120 of the first and second impellers 34 and 36are about equal in size or diameter. The vanes 112 of the first impeller34 are longer than the corresponding vanes 122 of the second impeller36. The configurations of the first and second impellers 34 and 36 inthe embodiment of FIGS. 1-4 illustrate one example impellerconfiguration. Those skilled in the art will appreciate that theimpellers 34 and 36 could have alternative configurations.

The back plates 110 and 120 have reduced diameters such that the vanes112 and 122, respectively, extend radially beyond their outer edges. Theback plates 110 and 120 are directly facing the left and right pumpinlets 46 and 56, respectively. Therefore, fluid pressures acting on theback plates 110 and 120 are primarily inlet pressures and thus exertforces on the rotor assembly 30 that are primarily axial, i.e., parallelto the axis 12. Outlet pressures produced by the blood pump 10 aregenerated primarily at the end portions of the vanes 112 and 122 thatare positioned radially beyond the outer diameter of the back plates 110and 120.

The blood pump 10 of the illustrated embodiment has a configuration thatdiffers from that of a conventional centrifugal pump design in two basicways. First, the blood pump 10 utilizes an open-vaned impeller with anunusually high axial clearance having non-symmetrical front and backaxial clearances (see FIGS. 2 and 3). Second, the radial vanes extendinto the volute section in a manner typical for a peripheral (orregenerative) pump. This extension creates a back-of-vane clearance forpassive performance modulation. Also, the rotor magnet 62, being shorterthan the stator core 26, allows for a controlled amount of free axialmovement of the rotating assembly 30.

It has been found that for constant system resistances, output flow andpump speed have a linear relationship. As a result, the controlalgorithm executed by the controller 82 adjusts pump speed to provide anominal systemic flow. Balanced systemic and pulmonary flows areachieved by adjusting of the axial position of the rotor assembly 30.According to the first embodiment of the invention, the axialadjustments of the rotor assembly 30 occur inherently or automaticallyas a result of the configurations of the left and right impellers 34 and36 and due to hydraulic pressures.

Because the axial hydrodynamic forces acting on the back plate portions110 and 120 of the impellers 34 and 36 are primarily those created bypump inlet pressures, the axial position of the rotor assembly 30adjusts in response to pressure differentials between the left and rightinlet portions 92 and 102. As the axial position of the rotor assembly30 adjusts, the geometry and hydraulic performance of the left and rightpumps 42 and 52 changes, as described above. This produces acorresponding change or adjustment in the outlet flows and pressures ofthe left and right pumps 42 and 52, trading pressure and flowperformance between the two pumps. The blood pump 10 is thus configuredwith a self-adjusting rotor assembly 30 that helps balance pulmonary andsystemic flows, as well as atrial pressures, through incremental changesthe hydraulic performance of the left and right pumps 42 and 52.

When operating in high clearance, minimum pump performance occurs whenthe pumping vanes are centered in the axial clearance (front and backclearances equal). Therefore, performance can be modulated by moving theimpellers 34 and 36 in either axial direction. In the self-balancingconfiguration of FIG. 2, maximum performance for the left pump 42 occurswhen back clearance A1 is minimum, while maximum performance for theright pump 52 occurs when back clearance A2 is minimum. The passivecontrol implemented in the embodiment of FIG. 2 modulates performance byadjusting the back clearances A1 and A2. The advantage of using the back(inside) edges to modulate performance is that hydraulic forcesoperating on the rotating assembly can enforce the correct direction ofaxial movement for passive control, thereby eliminating the need for anactive axial control system.

During operation of the blood pump 10 as TAH, pump speed can bemodulated at normal pulse rates to create pulsatile flow and pressure,simulating normal hemodynamics in the patient. For example, it was foundthat a ±30% speed modulation enforces a highly pulsatile condition.Further, the speed wave form can be adjusted to tailor thecharacteristics of the systemic pressure pulses to mimic the amplitudeand systolic/diastolic timing desired clinically.

Advantageously, since flow is directly related to current and speed, thecurrent wave form can be analyzed to determine any interruptions in flowduring each control cycle. This may, for example, help detect collapseof the left or right atria, in which case an incremental decrease inaverage speed or magnitude of the speed pulsation may be triggeredautomatically. Also, based on the motor current response to the speedand duty cycle, the patient's pulmonary and systemic pressures andvascular resistances can be estimated by calculation, allowing thesystem to be used as a continuous patient monitor.

A second embodiment of the present invention is illustrated in FIG. 6.The second embodiment of the invention is similar to the firstembodiment of the invention illustrated in FIGS. 1-5. Referring to FIG.6, the blood pump 200 has a two-stage centrifugal pump configurationsimilar to that of FIGS. 1-5. The blood pump 200 may thus be configuredfor use as a total artificial heart (TAH) device. The blood pump 200could, however, be suitable for non-TAH implementations, such asbiventricular support or any implementation in which a dual or two stagefluid handling pump with pressure balancing features is desired.

Referring to FIG. 6, the blood pump 200 includes a stator assembly 220,a rotor assembly 230, a left pump housing 240, and a right pump housing250. In the assembled condition, the rotor assembly 230 is supported bythe stator assembly 220 for rotation about an axis 212. The pumphousings 240 and 250 are fixed to the stator assembly 220 to enclose therotor assembly 230. The rotor assembly 230 includes a motor rotor 232, afirst or left impeller 234, and a second or right impeller 236.

The motor rotor 232 includes a core 260 upon which a ring-shapedpermanent motor magnet 262 is mounted. A fill material 264, such as alow density magnetically permeable material, may be used to help supportthe magnet 262 on the motor rotor 232. The left and right impellers 234and 236 are secured to the core 260 by known means, such as adhesives ormechanical fasteners. Alternatively, the impellers 234 and 236 could beformed (e.g., molded) as a single piece of material with the core 260.

The stator assembly 220 includes a stator housing 222 that supports amotor stator 224. The motor stator 224 includes a stator core and motorwindings, illustrated schematically at 226 and 228, respectively in FIG.6. The motor windings 228 are electrically connected to control wires270 of a control cable 272 that enters the stator housing 222 through aconduit 274 and is sealed by a potting material 276.

The blood pump 200, when assembled, includes a centrifugal first or leftpumping stage or pump 242. The left pump 242 includes the left impeller234 and a left pump chamber 244 in which the left impeller is disposed.The left pump chamber 244 is defined, at least partially, by the leftpump housing 240 and the stator assembly 220. The left pump 242 alsoincludes a left pump inlet 246 and a left pump outlet 248 that, in theillustrated embodiment, are formed as integral portions of the left pumphousing 240. The left pump housing 240 includes an inlet surface 290that helps define an inlet portion 292 of the left pump chamber 244 influid communication with the inlet 246. The left pump housing 240 alsoincludes a volute surface 294 that helps define a volute portion 296 ofthe left pump chamber 244 in fluid communication with the outlet 248.

The blood pump 200, when assembled, also includes a centrifugal secondor right pumping stage or pump 252. The right pump 252 includes theright impeller 236 and a right pump chamber 254 in which the rightimpeller is disposed. The right pump chamber 254 is defined, at leastpartially, by the right pump housing 250 and the stator assembly 220.The right pump 252 also includes a right pump inlet 256 and a right pumpoutlet 258 that, in the illustrated embodiment, are formed as integralportions of the right pump housing 250. The right pump housing 250includes an inlet surface 300 that helps define an inlet portion 302 ofthe right pump chamber 254 in fluid communication with the inlet 256.The right pump housing 250 also includes a volute surface 304 that helpsdefine a volute portion 306 of the right pump chamber 254 in fluidcommunication with the outlet 258.

The motor rotor 232 and motor stator 224 help define a motor 280 of theblood pump 200 that drives the left and right pumps 242 and 252. Themotor 280 may be any type of electric motor suited to drive the pumps242 and 252 and deliver the desired performance characteristics. Forexample, in the illustrated embodiment, the motor 280 may have amulti-phase brushless DC motor configuration. A motor controller 282 isoperative to excite the phase windings 228 of the motor 280 via thecable 272 to achieve desired performance of the motor portion, such asmotor speed or current. For example, the motor controller 282 applypulse width modulated voltage to the motor phases in order to achievethe desired the desired motor/pump performance.

During operation of the blood pump 200, the rotor assembly 230 rotatesabout the axis 212 relative to the stator assembly 220. The rotorassembly 230 is supported or rides on a hydrodynamic or fluid filmbearing formed by the pumped fluid, i.e., blood. Alternatively, theblood pump 200 could include other types of bearing features, such asmechanical bearings or bearing surfaces formed from or coated with lowfriction materials, for facilitating rotation of the rotor assembly 230.As a further alternative, the rotor assembly 230 could be magneticallysuspended.

The materials used to construct the blood pump 200 may be formed frommaterials conducive to blood pumping implementations. For example,portions of the blood pump 200 that are exposed to blood flow duringuse, such as the impellers 234 and 236 and pump housings 240 and 250,may be formed from, coated, or encased in a biocompatible material, suchas stainless steel, titanium, ceramics, polymeric materials, compositematerials, or a combination of these materials. Surfaces or portions ofthe blood pump 200 that may contact each other during use, such as theleft impeller 234 and pump housing 240 or the right impeller 236 andpump housing 250, may also be formed or coated with low frictionmaterials, such as fluorocarbon polymer coatings, diamond-like carboncoatings, ceramics, titanium, and diamond coated titanium.

In FIG. 6, arrows are used to illustrate the blood pump 200 in a totalartificial heart (TAH) implementation in which the pump takes over thefunction of a patient's heart (not shown). In this configuration, theleft pump inlet 246 is connected with the left atrium, the left pumpoutlet 248 is connected to the aorta, the right pump inlet 256 isconnected to the right atrium, and the right pump outlet 258 isconnected to the pulmonary artery. In operation, the left pump 242delivers oxygenated blood to the aorta from the left atrium and theright pump 252 delivers deoxygenated blood to the pulmonary artery fromthe right atrium.

According to the present invention, the blood pump 200 balances systemicand pulmonary pressures and flow rates by adjusting the geometry orconfiguration of the left (systemic) pump 242 and right (pulmonary) pump252. The blood pump 200 is configured with a clearance that permitsaxial movement of the rotor assembly 230 relative to the stator assembly220. In FIG. 6, the rotor assembly 230 is positioned about midpoint inthis axial clearance, leaving an axial clearance between the leftimpeller 234 and the left pump housing 240, identified generally at“B1,” and an axial clearance between the right impeller 236 and theright pump housing 250, identified generally at “B2.” During operationof the blood pump 200, the rotor assembly 230 can move or shuttleaxially relative to the stator assembly 220 due to electromotive forceof an actuator 350, such as an electric solenoid, that is connected tothe controller 282 via the cable 272. The rotor assembly 230 can moveaxially between a left position, in which the left impeller 234 ispositioned adjacent or engaging the left pump housing 240, and a rightposition, in which the right impeller 236 is positioned adjacent to theright pump housing 250.

When the rotor assembly 230 moves axially between the left and rightpositions, the configurations or geometries of the left and right pumps242 and 252 are altered. As the axial position of the left impeller 234changes, the clearance B1 between the left impeller and the left pumphousing 240 changes, which alters the volume of the left pump chamber244 and the configuration or geometry of the left pump 242. Similarly,as the axial position of the right impeller 236 changes, the clearanceB2 between the right impeller and the right pump housing 250 changes,which alters the volume of the right pump chamber 254 and theconfiguration or geometry of the right pump 252.

As the clearances B1 and B2 increase, the first and second pumps 242 and252 reduce hydraulic output. Thus, for a given pump speed, as theimpellers 234 and 236 move toward their respective pump housings 240 and250 (i.e., reducing their respective clearances B1 and B2), the pumps242 and 252 increase pressure and flow increase accordingly. Conversely,as the impellers 234 and 236 move away from their respective pumphousings 240 and 250 (i.e., increasing their respective clearances B1and B2), the pumps 242 and 252 decrease pressure and flow decreaseaccordingly.

It will thus be appreciated that, for the single motor, two-stageconfiguration of the blood pump 200 of the present invention, axialmovement of the rotor assembly 230 that produces increased pressure andflow at the left pump stage 242 will also produce a decrease in pressureand flow at the right pump stage 252. Similarly, axial movement of therotor assembly 230 that produces increased pressure and flow at theright pump stage 252 will also produce a decrease in pressure and flowat the left pump stage 242. From this, it follows that, for any givenspeed of the blood pump 200, the pressures and flows of the left andright pump stages 242 and 252 can be balanced if the axial position ofthe rotor assembly 230 relative to the stator assembly 220 is adjustedto the proper position.

Based on this principle, using the blood pump 200, systemic andpulmonary pressure and flow characteristics can be controlled throughadjusting the axial position of the rotor assembly 230. According to thesecond embodiment of the present invention, the blood pump 200 isconfigured for active control of the axial position of the of the rotorassembly 230 and, thus, the geometry or configuration of the left andright pumps 242 and 252.

It has been found that, for constant system resistances, output flow andpump speed have a linear relationship. It has also been found that, fora given pump speed, there is an electrical power level, obtained byadjusting the axial position of the rotor assembly 230, that correspondswith balanced flows at the left pump 242 and right pump 252. As aresult, the control algorithm executed by the controller 282 adjustspump speed to provide a nominal systemic flow, while balanced systemicand pulmonary flows are achieved by adjusting of the axial position ofthe rotor assembly 230. According to the second embodiment of theinvention, the axial adjustments of the rotor assembly 230 relative tothe stator assembly 220 are achieved through the use of anelectromechanical actuator 350, such as a solenoid, that is connected tothe controller 282 via the cable 272. The solenoid 350 is actuatable toone of two positions: a first or left position and a second or rightposition. In the left position, the solenoid 350 causes the axialposition of the rotor assembly 230 to shift to a first or left position,in which the left impeller 234 is positioned adjacent or near the inletsurface 290 of the left pump housing 240, effectively increasing thehydraulic output of the left pump stage 242 and decreasing the hydraulicoutput of the right pump stage 252, as described above. In the rightposition, the solenoid 350 causes the axial position of the rotorassembly 230 to shift to a second or right position, in which the rightimpeller 236 is positioned adjacent or near the inlet surface 300 of theright pump housing 250, effectively increasing the hydraulic output ofthe right pump stage 252 and decreasing the hydraulic output of the leftpump stage 252, as described above.

The solenoid 350 may be configured to place the rotor assembly 230 inthe left and right positions in a variety of manners. For example, thesolenoid 350 may be a latching solenoid. In this configuration, thesolenoid 350 may include two separate coils 352, one for selecting theleft position and one for selecting the right position, fixed to thestator assembly 220 and an armature 354, such as one or more magnets,fixed to the rotor assembly 230. In this latching configuration, thesolenoid 350 includes a magnetic latching mechanism that maintains therotor assembly 230 in the selected position without constant applicationof power to the solenoid. In operation, the coils 352 may be energizedby a short current pulse of sufficient magnitude and duration to movethe armature 354, and thus the rotor assembly 230, to the desiredleft/right position. At this point, the latching mechanism is actuatedand maintains the rotor 230 at the desired position. When the oppositecoil is energized, the latching mechanism releases the rotor assembly230 to move to the opposite position under the pull of the coil 352 onthe armature 354. The mechanism then latches magnetically, thusmaintaining the axial position of the rotor assembly 230 when the coil352 is de-energized.

In an alternative configuration, the solenoid 350 may be a ratcheting ortoggle-type latching solenoid configured for pulse-left/pulse-rightoperation. In this configuration, the solenoid 350 may include a singlecoil and latch mechanism that, when the coil is energized, latches therotor assembly alternately in the left and right positions. Thus, duringoperation, if the rotor assembly is in the right position, the nextenergy pulse will place the rotor assembly in the left position. Thenext energy pulse will then place the rotor assembly in the rightposition, and so on.

In another alternative configuration, the solenoid 350 may be anon-latching, continuous current solenoid. In this configuration, thesolenoid may include a single coil for moving an armature that is springbiased to one of the left and right positions. When the coil isde-energized, the spring maintains the armature and thus the rotor, atone of the left and right positions. When the coil is energized, thearmature and rotor are moved against the spring bias to the oppositeposition. The armature and rotor are maintained at this position untilthe coil is de-energized, at which time the spring moves the armatureand rotor back to the original position.

In operation of the blood pump 200, motor speed is modulated at normalpulse rates to create pulsatile flow and pressure. Balanced systemic andpulmonary flow and atrial pressure balance are achieved through activeadjustments of the axial position of the rotor assembly 230 via thesolenoid 350 to adjust the hydraulic performance of the left and rightpumps 242 and 252. These balanced flows and pressures are achieved bysplitting the control cycle (e.g., 10 seconds) between the left andright positions. Left and right flow will be estimated from the speed,power consumption, and the change in power consumption as the rotorassembly 230 toggles between the left and right axial positions.

In operation, the axial position of the rotor assembly 230 is toggledback and forth between the left and right positions during the controlcycle (e.g., ten seconds) of the pump 200. As the axial position of therotor assembly 230 toggles, the geometry and hydraulic performance ofthe left and right pumps 242 and 252 changes, as described above. Thisproduces a corresponding net change or adjustment in the outlet flowsand pressures of the left and right pumps 242 and 252, increasing theoutlet flow and pressure on one side of the pump and decreasing theoutlet flow and pressure at the opposing side of the pump. The bloodpump 200 and the controller 282 are thus configured to balance pulmonaryand systemic flows, as well as atrial pressures, through incrementalchanges in the hydraulic performance of the left and right pumps 242 and252.

The active control embodiment of the blood pump 200 of FIG. 6 uses frontvane clearance to modulate performance. This has two potentialadvantages. First, right/left performance bias can be controlledexternally at the expense of more complexity. Second, the total axialclearance is less, allowing better pump efficiency. Also, the rotormagnet 262 is shorter than the stator core 226 to allow a controlledamount of free axial movement of the rotating assembly 230.

During operation of the blood pump 200, left and right atrial pressuresequilibrate to within several mmHg. As the flow approaches equilibrium,trending in the current draw of the pump 200 indicates the direction ofadjustment for fine-tuning the duty cycle. Also, pump speed can bemodulated at normal pulse rates to create pulsatile flow and pressureand stable hemodynamics in the patient. For example, it was found that a±30% speed modulation enforces a highly pulsatile condition. Further,the speed wave form can be adjusted to tailor the characteristics of thesystemic pressure pulses to mimic the amplitude and systolic/diastolictiming desired clinically.

Advantageously, since flow is approximately related to current andspeed, the current wave form can be analyzed to determine anyinterruptions in flow during each control cycle. This may, for example,help detect collapse of the left or right atria, in which case anincremental decrease in average speed or magnitude of the speedpulsation may be triggered automatically. Also, based on the speed andduty cycle, the patient's pulmonary and systemic pressures and vascularresistances can be estimated by calculation, allowing the system to beused as a continuous patient monitor.

A blood pump 400 according to a third embodiment of the presentinvention is illustrated in FIG. 7. The blood pump 400 of FIG. 7 has aconfiguration that is similar to the embodiment of FIG. 6, except thatthe embodiment of FIG. 7 includes a rotor assembly 410 that does notmove axially to alter the pump geometry during operation. In thisconfiguration, the rotor magnet 420 is the same length or longer thanthe stator core 422, which magnetically constrains the axial position ofthe rotor assembly 410.

The blood pump 400 of FIG. 7 may be particularly well-suited for use asa ventricular assist device (VAD), such as a bi-ventricular assistdevice (BiVAD) that combines right ventricular assist device (RVAD) andleft ventricular assist device (LVAD) functions in a single pump. Withan RVAD, the total pulmonary artery flow is shared between the VAD andnative ventricle, so precise right/left pump control is not as criticalas for a total artificial heart. It has been found that performancecharacteristics can be crafted into the pumping element design, whichcan allow a degree of completely passive regulation of a BiVAD system.In this embodiment, the configurations and geometries of the left pump442 (LVAD) and right pump 452 (RVAD) may be designed to have pressureversus flow characteristics similar to those shown in FIG. 8. As shownin FIG. 8, the left pump 442 has a pressure rise that decreases sharplywith increasing flow, causing the left flow to be primarily a functionof speed. The right pump 452 has a characteristic pressure rise that isa function of speed, and relatively independent of flow. In this way,the left pump 442 acts as a flow regulator for systemic flow, while theright pump 452 acts as a differential pressure regulator for moderateunloading the right ventricle.

A blood pump 500 according to a fourth embodiment of the presentinvention is illustrated in FIGS. 9-12. The blood pump 500 of FIGS. 9-12has a two-stage or dual centrifugal pump configuration that is similarto the embodiments of FIGS. 1-5 and 7. The blood pump 500 may thus beconfigured for use as a total artificial heart (TAH) device. The bloodpump 500 could, however, be suitable for non-TAH implementations, suchas biventricular support or any implementation in which a dual or twostage fluid handling pump with pressure balancing features is desired.

Referring to FIGS. 9-11, the blood pump 500 includes a stator assembly520, a rotor assembly 530, a left pump housing 540, and a right pumphousing 550. In an assembled condition of the blood pump 500, the rotorassembly 530 is supported by the stator assembly 520 for rotation aboutan axis 512. The pump housings 540 and 550 are fixed to the statorassembly 520 to enclose the rotor assembly 530. The rotor assembly 530includes a motor rotor 532, a first or left impeller 534, and a secondor right impeller 536.

The motor rotor 532 includes a core 560 (FIG. 12) surrounded orotherwise encased in a shell or casing 564 upon which a ring-shapedpermanent magnet 562 is mounted. The core 560 may be constructed of alow density magnetically permeable material, may be used to help supportthe magnet 562 on the motor rotor 532, thereby allowing a neutralbuoyancy rotating assembly and insensitivity to the attitude of the pumpassembly. The left and right impellers 534 and 536 may be secured to thecore 560 by known means, such as adhesives or mechanical fasteners, or,as shown in FIGS. 9-11, could be formed (e.g., molded) as a single pieceof material with the shell 564.

The stator assembly 520 includes a stator housing 522 that supports amotor stator 524. The motor stator 524 includes a stator core and motorwindings, illustrated schematically at 526 and 528, respectively in FIG.12. The motor windings 528 are electrically connected to control wires570 of a control cable 572 that enters the stator housing 522 through aconduit 574 and a strain relief material 576.

The blood pump 500, when assembled, includes a centrifugal first or leftpumping stage or pump 542. The left pump 542 includes the left impeller534 and a left pump chamber 544 in which the left impeller is disposed.The left pump chamber 544 is defined, at least partially, by the leftpump housing 540 and the stator assembly 520. The left pump 542 alsoincludes a left pump inlet 546 and a left pump outlet 548 that, in theillustrated embodiment, are formed as integral portions of the left pumphousing 540. The left pump housing 540 includes an inlet surface 590that helps define an inlet portion 592 of the left pump chamber 544 influid communication with the inlet 546. The left pump housing 540 alsoincludes a volute surface 594 that helps define a volute portion 596 ofthe left pump chamber 544 in fluid communication with the outlet 548.

The blood pump 500, when assembled, also includes a centrifugal secondor right pumping stage or pump 552. The right pump 552 includes theright impeller 536 and a right pump chamber 554 in which the rightimpeller is disposed. The right pump chamber 554 is defined, at leastpartially, by the right pump housing 550 and the stator assembly 520.The right pump 552 also includes a right pump inlet 556 and a right pumpoutlet 558 that, in the illustrated embodiment, are formed as integralportions of the right pump housing 550. The right pump housing 550includes an inlet surface 600 that helps define an inlet portion 602 ofthe right pump chamber 554 in fluid communication with the inlet 556.The right pump housing 550 also includes a volute surface 604 that helpsdefine a volute portion 606 of the right pump chamber 554 in fluidcommunication with the outlet 558. The right pump housing 550 furtherincludes a chamber 608 adjacent the volute portion 606 into which theright impeller 536 enters as the rotor assembly 530 moves axially to theright as viewed in FIG. 12. The right impeller 536 leaves the voluteportion 606 as it enters the chamber 608.

The motor rotor 532 and motor stator 524 help define a motor 580 of theblood pump 500 that drives the left and right pumps 542 and 552. Themotor 580 may be any type of electric motor suited to drive the pumps542 and 552 and deliver the desired performance characteristics. Forexample, in the illustrated embodiment, the motor 580 may have a singlephase or multi-phase brushless, sensorless DC motor configuration. Amotor controller (not shown) is operative to excite the phase windings528 of the motor 580 via the cable 572 to achieve desired performance ofthe motor portion, such as motor speed or current. For example, themotor controller may apply pulse width modulated voltage to the motorphases in order to achieve the desired motor/pump performance.

Referring to FIG. 11, the first impeller 534 includes a back plate 610and a plurality of vanes 612 that extend radially from the rotor 530.The vanes 612 include first or primary vanes 614 and second or splittervanes 616, the splitter vanes being shorter than the primary vanes. Inthe embodiment illustrated in FIGS. 9-12, there are two splitter vanes616 positioned between pairs of primary vanes 614. The vanes 612 areconfigured with a low incidence inlet and a radial discharge.

The second impeller 536 includes a back plate 620 and a plurality ofvanes 622 that extend radially along the end face of the rotor 530. Thevanes 622 include first or primary vanes 624 and second or splittervanes 626, the second vanes being shorter than the first vanes. In theembodiment illustrated in FIGS. 9-12, the primary vanes 624 and splittervanes 626 are arranged in an alternating fashion about the rotor 530.The vanes 622 are configured with a low incidence inlet and a radialdischarge.

The vanes 612 of the first impeller 534 are longer than thecorresponding vanes 622 of the second impeller 536. The configurationsof the first and second impellers 534 and 536 in the embodiment of FIGS.9-12 illustrate one example impeller configuration. Those skilled in theart will appreciate that the impellers 534 and 536 could havealternative configurations.

The back plates 610 and 620 are aligned axially with the left and rightpump inlets 546 and 556, respectively. Therefore, fluid pressures actingon the back plates 610 and 620 are primarily inlet pressures and thusexert forces on the rotor assembly 530 that are primarily axial, i.e.,parallel to the axis 512. Outlet pressures produced by the blood pump500 are generated primarily at the end portions of the vanes 612 and622. The vanes 612 of the first impeller 534 extend radially beyond theouter diameter of the back plate 610.

During operation of the blood pump 500, the rotor assembly 530 rotatesabout the axis 512 relative to the stator assembly 520. The rotorassembly 530 is supported or rides on a hydrodynamic or fluid filmbearing formed by the pumped fluid, i.e., blood. Alternatively, theblood pump 500 could include other types of bearing features, such asmechanical bearings or bearing surfaces formed from or coated with lowfriction materials, for facilitating rotation of the rotor assembly 530.As a further alternative, the rotor assembly 530 could be magneticallysuspended.

The materials used to construct the blood pump 500 may be formed frommaterials conducive to blood pumping implementations. For example,portions of the blood pump 500 that are exposed to blood flow duringuse, such as the impellers 534 and 536 and pump housings 540 and 550,may be formed from, coated, or encased in a biocompatible material, suchas stainless steel, titanium, ceramics, polymeric materials, compositematerials, or a combination of these materials. Surfaces or portions ofthe blood pump 500 that may contact each other during use, such as theleft impeller 534 and pump housing 540, the right impeller 536 and pumphousing 550, or the rotor casing 564, may also be formed or coated withlow friction materials, such as a fluorocarbon polymer coatings,diamond-like carbon coatings, ceramics, titanium, and diamond coatedtitanium.

Those skilled in the art will appreciate that, in a TAH scenario, it isimportant to balance pulmonary and systemic arterial blood flows andatrial pressures. For example, if the right pump 552 delivers blood at ahigher flow rate than the left pump 542, blood may accumulate in thelungs and can lead to congestive heart failure. As another example, ifthe left pump 542 delivers blood at a higher flow rate than the rightpump 552, blood may accumulate in the liver and can lead to liverfailure. The goal for the blood pump 500 is thus to balance pulmonaryand systemic arterial blood flows and atrial pressures. According to thepresent invention, the blood pump 500 balances systemic and pulmonaryatrial pressures and arterial flow rates by adjusting the geometry orconfiguration of the left (systemic) pump 542 and right (pulmonary) pump552.

According to the present invention, the blood pump 500 is configuredwith a clearance that permits axial movement of the rotor assembly 530relative to the stator assembly 520. Referring to FIG. 12, the rotorassembly 530 is positioned about midpoint in this axial clearance. Theblood pump 500 has an axial back clearance between the left impeller 534and the left pump housing 540 identified generally at “D1.” As shown inFIG. 12, D1 is the clearance between the vanes 612 of the left impeller534 and a back surface 578 of the left pump chamber 544, which may bedefined at least partially by the stator assembly 520, the left pumphousing 540, or both the stator assembly and the left pump housing.During operation of the pump 500, when the rotor assembly 530 movesaxially relative to the stator assembly 520, the left impeller 534 movesaxially within the left pump chamber 544.

The blood pump 500 has an axial front clearance between the rightimpeller 536 and the right pump housing 550, identified generally at“D2.” The front clearance D2 is defined between the back plate 620 ofthe right impeller 536 and an annular ridge 630 on the right pumphousing 550 where the volute surface 604 intersects the surface definingthe chamber 608. The clearance D2 is indicative of the degree to whichthe vanes 622 of the second impeller 536 extend into the chamber 608 andout of the volute chamber 606. The clearance D2 is also indicative ofthe size of an annular opening or aperture 632 defined between the backplate 620 and the ridge 630. The aperture 632 defines the area throughwhich the second impeller 536 pumps fluid through the volute chamber606. As D2 decreases, the area of the aperture 632 decreases as thevanes 622 of the second impeller 536 move or extend further out of thevolute chamber 606 into the chamber 608. Conversely, as D2 increases,the area of the aperture 632 increases as the vanes 622 of the secondimpeller 536 move or extend further out of the chamber 608 into thevolute chamber 606.

In the configuration shown in FIG. 12, left pump 542 performanceimproves as D1 decreases and right pump 552 performance improves as D2increases. During operation of the blood pump 500, the rotor assembly530 can move or shuttle axially relative to the stator assembly 520 dueto hydrodynamic pumping forces created by the left and right pumps 542and 552. The rotor assembly 530 can move axially between a leftposition, in which D1 and D2 are maximum, and a right position, in whichD1 and D2 are minimum.

When the rotor assembly 530 moves axially between the left and rightpositions, the configurations or geometries of the left and right pumps542 and 552 are altered. As the axial position of the left impeller 534changes, the clearance D1 between the left impeller and back surface 578of the left pump housing 540 changes, which alters the configuration andgeometry of the left pump 542 and left pump chamber 544. As the axialposition of the right impeller 536 changes, the clearance D2 between theright impeller and the right pump housing 550, which alters the size ofthe aperture 632, the configuration and geometry of the right pumpchamber 554, and the configuration or geometry of the right pump 552.

As the D1 clearance increases and the D2 clearance decreases, the firstand second pumps 542 and 552 decrease hydraulic output. Thus, for agiven pump speed, as the impellers 534 and 536 move toward the statorassembly 522 (i.e., reducing D1 and increasing D2), the pumps 542 and552 increase hydraulic output and pressure and flow increaseaccordingly. Conversely, as the impellers 534 and 536 move away from thestator assembly 522 (i.e., increasing D1 and decreasing D2), the pumps542 and 552 decrease hydraulic output and pressure and flow decreaseaccordingly.

It will thus be appreciated that, for the single motor, two-stageconfiguration of the blood pump 500 of the present invention, axialmovement of the rotor assembly 530 that produces increased pressure andflow at the left pump stage 542 will also produce a decrease in pressureand flow at the right pump stage 552. Similarly, axial movement of therotor assembly 530 that produces increased pressure and flow at theright pump stage 552 will also produce a decrease in pressure and flowat the left pump stage 542. From this, it follows that, for any givenspeed of the blood pump 500, the pressures and flows of the left andright pump stages 542 and 552 can be balanced if the axial position ofthe rotor assembly 530 relative to the stator assembly 520 is adjustedto the proper position.

Based on this principle, using the blood pump 500, systemic andpulmonary pressure and flow characteristics can be controlled throughadjusting the axial position of the rotor assembly 530. In theembodiment of FIGS. 9-12, the axial position of the of the rotorassembly 530 and, thus, the geometry or configuration of the left andright pumps 542 and 552 can is controlled passively.

In the passive control configuration of the blood pump 500, the axialposition of the rotor assembly 530 is controlled passively or inherentlythrough hydraulic forces created by the left and right pumps 542 and 552during operation.

In operation, the control algorithm executed by the controller adjustspump speed to provide a nominal systemic flow. Balanced systemic andpulmonary flows are achieved by adjusting of the axial position of therotor assembly 530. The axial adjustments of the rotor assembly 530occur inherently or automatically as a result of the configurations ofthe left and right impellers 534 and 536 and due to hydraulic pressures.Referring to FIG. 13, the control of speed of the pump 500 is based uponthe characteristic mathematical relationship between speed, electricpower consumption, and equilibrium output flow. In FIG. 13, net wattsare equal to the electric power supplied to the motor minus the bearingdrag power/motor efficiency and is calculated as the console power minusthe power required to run the motor without impellers, where SystemicVascular Resistance (SVR)=500-2000 dyne-sec/cm⁵ and Pulmonary VascularResistance (PVR)=100-500 dyne-sec/cm⁵. Also, in FIG. 13, KRPM is motorrpm/1000. The current response to speed pulses will also allowestimation of systemic vascular resistance, which can be correlated tothe change in power consumption with speed.

Because the axial hydrodynamic forces acting on the back plate portions610 and 620 of the impellers 534 and 536 are primarily those created bypump inlet pressures, the axial position of the rotor assembly 530adjusts in response to pressure differentials between the left and rightinlet portions 592 and 602. As the axial position of the rotor assembly530 adjusts, the geometry and hydraulic performance of the left andright pumps 542 and 552 changes, as described above. This produces acorresponding change or adjustment in the outlet flows and pressures ofthe left and right pumps 542 and 552, trading pressure and flowperformance between the two pumps. The blood pump 500 is thus configuredwith a self-adjusting rotor assembly 530 that helps balance pulmonaryand systemic flows, as well as atrial pressures, through incrementalchanges the hydraulic performance of the left and right pumps 542 and552.

When operating in high clearance, minimum pump performance occurs whenthe pumping vanes are centered in the axial clearance (front and backclearances equal). Therefore, performance can be modulated by moving theimpellers 534 and 536 in either axial direction. Maximum performance forthe left pump 542 occurs when back clearance D1 is minimum, whilemaximum performance for the right pump 552 occurs when front clearanceD2 is maximum. The passive control implemented in the embodiment ofFIGS. 9-12 modulates performance by adjusting the clearances D1 and D2.The advantage of using the back (inside) edges to modulate performanceis that hydraulic forces operating on the rotating assembly can enforcethe correct direction of axial movement for passive control, therebyeliminating the need for an active axial control system.

In the embodiment of FIGS. 9-12, the left pump 542 is configured to havea steep pressure rise vs. flow characteristic and also to regulateperformance via the impeller vane clearance D1 such that the left pumpoutput increases as the rotating assembly moves to the right as viewedin FIG. 12. The right pump 552 is configured to regulate performance bycreating an aperture 632 that controls impeller vane discharge,decreasing output as the rotating assembly moves to the right (in FIG.12), and increasing output as the rotating assembly moves to the left.

Advantageously, the configuration is self-regulating. In response to achanging vascular resistance, the rotating rotor assembly 530 moves inthe direction of lowest inlet pressure to automatically correctimbalances between the inlet pressures at the left and right inlets 546and 556. Thus, for example, in the case of inlet obstruction due to leftatrial suction, the left inlet pressure drops and the rotating assemblymoves to the left, i.e., in the direction of low pressure. This resultsin decreased left pump performance simultaneous with increased rightpump performance, which automatically corrects the suction condition.The pump 500 would operate similarly and correspondingly to selfregulate in the event of right atrial suction.

From the above description of the invention, those skilled in the artwill perceive improvements, changes and modifications. Suchimprovements, changes and modifications within the skill of the art areintended to be covered by the appended claims.

1. A pump comprising: a housing; a stator supported in the housing; anda rotor assembly comprising: a rotor supported in the housing forrotation relative to the stator about an axis; a first impelleroperatively coupled to a first axial end of the rotor for rotation withthe rotor about the axis; and a second impeller operatively coupled to asecond axial end of the rotor, opposite the first axial end, forrotation with the rotor about the axis; the rotor assembly being movablealong the axis relative to the housing to adjust hydraulic performancecharacteristics of the pump.
 2. The pump recited in claim 1, wherein therotor is of neutral buoyancy, the rotor assembly being insensitive topositional attitude due to the neutral buoyancy.
 3. The pump recited inclaim 1, wherein the housing comprises: a first pump housing part thathelps define a first pumping chamber in which the first impeller issupported for rotation about the axis, the first impeller being movablealong the axis relative to the first pump housing part; and a secondpump housing part that helps define a second pumping chamber in whichthe second impeller is supported for rotation about the axis, the secondimpeller being movable along the axis relative to the second pumphousing part; the first and second impellers when moved along the axiswith the rotor assembly moving relative to the first and second pumphousing parts to adjust the hydraulic performance characteristics of thepump.
 4. The pump recited in claim 1, further comprising an actuatoractuatable to adjust the axial position of the rotor assembly to achievedesired hydraulic performance characteristics of the pump.
 5. The pumprecited in claim 4, wherein the actuator comprises an electric solenoid.6. The pump recited in claim 3, wherein the first and second impellerseach comprise a back plate and a plurality of vanes extending radiallyfrom the back plate, the vanes having end portions that extend radiallybeyond an outer periphery of the back plate.
 7. The pump recited inclaim 6, wherein: the first back plate is aligned axially with anddirectly faces a first pump inlet of the first pump housing part suchthat fluid pressures acting on the first back plate are primarily inletpressures that exert axial hydraulic forces on the rotor assembly in afirst direction along the axis; and the second back plate is alignedaxially with and directly faces a second pump inlet of the second pumphousing part such that fluid pressures acting on the second back plateare primarily inlet pressures that exert axial hydraulic forces on therotor assembly in a second direction along the axis opposite the firstdirection.
 8. The pump recited in claim 7, wherein the axial position ofthe rotor assembly in the housing varies in response to the axial forcesexerted on the rotor assembly.
 9. The pump recited in claim 7, whereinfor a given pump speed, there is an axial position of the rotor assemblyat which the axial forces exerted on the first and second back platesare equal and opposite.
 10. The pump recited in claim 6, wherein: outletpressures produced by the first impeller are generated primarily at theend portions of the vanes of the first impeller; and outlet pressuresproduced by the second impeller are generated primarily at the endportions of the vanes of the second impeller.
 11. The pump recited inclaim 6, wherein the vanes of the first and second impellers compriseprimary vanes and splitter vanes with low incidence inlets and radialdischarges.
 12. The pump recited in claim 1, wherein the pump isconfigured such that fluid inlet pressures acting on the first impellerexerts an axial force on the rotor assembly in a first direction alongthe axis and fluid inlet pressures acting on the second impeller exertsan axial force on the rotor assembly in a second direction along theaxis, opposite the first direction.
 13. The pump recited in claim 12,wherein the axial forces exerted on the rotor assembly by the fluidinlet pressures acting on the first and second impellers adjusts theaxial position of the rotor assembly to help balance the fluid inletpressures acting on the first and second impellers.
 14. The pump recitedin claim 1, wherein the hydraulic performance of the pump is adjusted byadjusting a back clearance between impeller vanes of at least one of thefirst and second impellers and a back surface of the housing.
 15. Thepump recited in claim 1, wherein the hydraulic performance of the pumpis adjusted by moving at least one of the first and second impellers outof a pumping chamber and into an adjacent chamber in response to theaxial movement of the rotor relative to the housing.
 16. The pumprecited in claim 1, wherein the hydraulic performance of the pump isadjusted by adjusting the size of a pumping aperture in response to theaxial movement of the rotor relative to the housing.
 17. A pumpcomprising: a motor comprising a stator and a rotor rotatable about anaxis relative to the stator; a first pump stage comprising a first pumphousing and a first impeller rotatable with the rotor about the axis inthe first pump housing; and a second pump stage comprising a second pumphousing and a second impeller rotatable with the rotor about the axis inthe second pump housing; the pump being adapted to adjust the axialposition of the first impeller in the first housing and the axialposition of the second impeller in the second housing to adjusthydraulic performance characteristics of the first and second pumpstages in response to inlet pressure differentials between the first andsecond pump stages.
 18. A pump comprising: a motor comprising a statorand a rotor rotatable about an axis relative to the stator; a first pumpstage comprising a first pump housing and a first impeller rotatablewith the rotor about the axis in the first pump housing; and a secondpump stage comprising a second pump housing and a second impellerrotatable with the rotor about the axis in the second pump housing; thefirst pump stage being configured to have a pressure rise that decreasessharply with increasing flow, the first pump stage flow thus beingprimarily a function of pump speed; the second pump stage beingconfigured to have a pressure rise that is primarily a function of pumpspeed and substantially independent of flow.
 19. A pump comprising: ahousing defining first and second pump housings; a rotor supported inthe housing and rotatable about an axis, the rotor comprising a firstimpeller disposed in the first pump housing and a second impellerdisposed in the second pump housing; the pump being configured such thatinlet pressures acting on the first impeller move the rotor relative tothe housing in a first direction along the axis and inlet pressuresacting on the second impeller move the rotor relative to the housing ina second direction along the axis opposite the first direction.
 20. Thepump recited in claim 19, wherein the first pump housing and the firstimpeller help define a first pumping stage of the pump and the secondpump housing and the second impeller help define a second pumping stageof the pump, movement of the rotor along the axis altering hydraulicperformance characteristics of the first and second pumping stages. 21.A pump comprising: a housing comprising a pumping chamber; a rotorsupported in the housing and rotatable about an axis, the rotorcomprising an impeller at least partially disposed in the pumpingchamber, the rotor being movable relative to the housing in an axialdirection parallel to the axis; the pump being configured such thataxial movement of the rotor causes the impeller to move axially betweenthe pumping chamber and an adjacent chamber to alter the hydraulicperformance of the pump.